1. Field of the Invention
The present invention relates to a mask pattern suitable for an exposure apparatus for exposing a photosensitive substrate with a mask pattern used in a photolithography step of manufacturing, e.g., semiconductor elements, and a photosensitive substrate alignment technique.
2. Related Background Art
In a photolithography step of manufacturing microdevices such as a semiconductor device, a liquid crystal display element, an image pickup element (CCD), and a thin film magnetic head, a projection exposure apparatus is used. In this apparatus, the image of a photomask or reticle (to be referred to as a xe2x80x9creticlexe2x80x9d hereinafter) on which a transfer pattern is formed is projected on a photosensitive substrate (e.g., a wafer or a glass plate coated with a photoresist) through a projection optical system.
In the projection exposure apparatus of this type, the reticle and the wafer must be aligned to each other with a high precision prior to an exposure process. To perform this alignment, a position detection mark (alignment mark) is formed (transferred by exposure) on the wafer in the first photolithography step, and an accurate position of the wafer (or a circuit pattern on the wafer) can be detected by detecting the position of this alignment mark.
There are three major methods of detecting the position of a mark on a wafer. In the first method, an mark image is picked up to detect its position by image processing. The second method is a laser beam scanning scheme in which a grating-like alignment mark with a pitch in a direction perpendicular to the measurement direction is scanned relative to a sheet beam to detect the position of the mark based on a change in the intensity of a beam scattered or diffracted by this mark. The third method is called a xe2x80x9cgrating alignmentxe2x80x9d method using a grating-like alignment mark with a pitch in the measurement direction. This method is further subdivided into several methods in accordance with the arrangements of optical systems.
In the first arrangement, two laser beams are incident on an entire alignment mark from the directions of particular diffracted orders to detect the mark position based on a change in the amount of the beam diffracted by the mark. This method includes a heterodyning scheme and a homodyning scheme. In the former scheme, a slight frequency difference is generated between two laser beams to move interference fringes formed by this difference at a constant speed. In the latter scheme, an alignment mark is moved with respect to still interference fringes formed by two laser beams having no frequency difference.
In the second arrangement, one laser beam is incident on an entire alignment mark, two beams diffracted by this mark are condensed on a xe2x80x9creference gratingxe2x80x9d and formed into an image, and the alignment mark and the reference grating are scanned relative to each other to detect the position of the mark based on a change in the amount of a transmitted beam or a reflected beam by the reference grating.
Although a position detection optical system can also employ an off-axis scheme in which an exclusive microscope is arranged in addition to a projection optical system, a TTL (Through The Lens) scheme using a projection optical system itself as a position detection optical system is generally excellent in detection stability. Normally, a projection optical system is optimumly designed for an exposure wavelength (quasimonochromatic ultraviolet ray) and has a large chromatic aberration with respect to a position detection beam (its wavelength is normally 500 nm or more). For this reason, in the TTL scheme, the position detection beam is limited to a monochromatic beam or a quasimonochromatic beam. To the contrary, a method in which position detection optical systems are separately arranged for detection beams having a plurality of wavelengths to perform position detection with the plurality of wavelengths even in the TTL scheme, and a method in which an optical system for correcting a chromatic aberration of a projection optical system is arranged, and a detection beam with a certain wavelength width is used have been attempted.
An alignment mark formed on the surface of, e.g., a wafer to be used in the above position detection methods is generally a corrugated mark with a difference in depth. This corrugated mark is slightly asymmetric due to processes such as etching and sputtering in a photolithography step, and coating nonuniformity of a photoresist. This asymmetry decreases the position detection precision.
Of these position detection schemes, particularly in the TTL scheme, the temporal coherence length of a detection beam is large because the detection beam is a monochromatic beam or a quasimonochromatic beam whose wavelength width is not so large. Therefore, in the asymmetric corrugated mark, the asymmetry in the direction of mark height (depth) adversely affects the position detection precision.
When an exclusive microscope is used, although a chromatic aberration is not limited in the wavelength band of a detection beam, its wavelength band is substantially limited within the range of 500 nm to 1,500 nm to prevent exposure of a photoresist on a wafer and by the transmittance of optical glass. For this reason, the temporal coherence length is 1 xcexcm or more, which becomes 1/1.66=0.60 xcexcm in a photoresist having a refractive index of, e.g., 1.66. As for an alignment mark whose difference in depth at a recessed or projecting portion is 0.60/2=0.30 xcexcm (the value is divided by 2 because the detection beam is reflected to reciprocate), the asymmetry in the direction of height (depth) also adversely affects the position detection precision even with the exclusive microscope.
When a detection beam having a wide wavelength is used in the TTL scheme, the precision is increased by averaging the wavelengths in an alignment mark with a difference in depth to a certain extent. In an alignment mark with a difference in depth smaller than the coherence length, however, the precision cannot be increased unlike in the above alignment mark.
If the position detection precision is decreased due to a difference in depth at a recessed or projecting portion and coating nonuniformity of a photoresist, the alignment (overlapping) precision between a reticle and a wafer is decreased in a photolithography step of manufacturing microdevices such as a semiconductor device, and particular in an exposure step. A microdevice with predetermined characteristics cannot be attained.
The present invention has been made in consideration of the above problem, and has as one of its object to provide a position detection mark and position detection method which realizes a high position detection precision, and a method of aligning a mask and a substrate with a high precision to form a microdevice on the substrate.
In the present invention, a difference h in depth of a recessed or projecting portion is set by
(2m+1)xcex/4xe2x88x920.05xcexxe2x89xa6hxe2x89xa6(2m+1)xcex/4+0.05xcex
where xcex is the wavelength or center wavelength of a monochromatic or quasimonochromatic detection light near a position detection mark (alignment mark) consisting of the recessed or projecting portion formed on a substrate. Therefore, in all the above position detection schemes, the detection error caused by the asymmetry in the direction of height (depth) of the position detection mark can be greatly reduced. Particularly, in the use of a grating mark (phase gratings having difference in depth) as the position detection mark, the amount of a beam diffracted by the mark is maximized under the above condition of the difference in depth, thereby further increasing the position detection precision.
This position detection mark is formed on the surface of a substrate or a film (layer) formed on the substrate. Particularly when the substrate is a semiconductor wafer, the position detection mark is desirably formed on the surface in the most initial step of processing the semiconductor wafer. Further, the position detection mark can desirably have an arrangement in which a plurality of recessed or projecting portions are aligned at a pitch in the measurement direction, an arrangement in which a plurality of projecting or recessed portions are aligned at a pitch in a direction almost perpendicular to the measurement direction, or an arrangement with a band-like projecting or recessed portion.
In addition, the image of the position detection mark may be picked up to detect the position or positional shift of the mark based on the image. The position or positional shift of the position detection mark may be detected based on a change in the amount of the beam scattered by the mark with relative scanning of the position detection mark and the detection beam. If the position detection mark is a grating, the mark and the detection beam (sheet beam or a pair of coherent beams) may be relatively scanned to detect the position or positional shift of the mark based on a change in the amount of the beam diffracted by the mark, or a pair of coherent beams having different frequencies are symmetrically incident on the position detection mark to detect the position or positional shift of the mark based on the phase of the photoelectrically converted signal of the beam diffracted by the mark. Still further, a coherent beam is irradiated on the position detection mark, at least two beams diffracted by the mark are formed into an image on a reference grating arranged at a position conjugated to the position detection mark, and the position detection mark and the reference grating are relatively scanned to detect the position or positional shift of the mark based on a change in the amount of the beam transmitted through or reflected by the reference grating.
Moreover, in the present invention, an alignment mark consisting of a recessed or projecting portion is formed on a substrate on which microdevices such as a semiconductor element, a liquid crystal display element, an image pickup element (CCD), and a thin film magnetic head are formed. At this time, the difference h in depth of the recessed or projecting portion is set to
(2m+1)xcex/4xe2x88x920.05xcexxe2x89xa6hxe2x89xa6(2m+1)xcex/4+0.05xcex
where xcex is the wavelength or center wavelength of a monochromatic or quasimonochromatic detection light near the alignment mark. The detection beam is irradiated on the alignment mark formed on the substrate covered with a photoresist to detect the position or positional shift of the mask. The substrate and the mask are relatively moved in accordance with the detection result to expose the photoresist formed on the substrate with the pattern image of the mask.
When the position detection mark is exposed in outer air (or vacuum), xcex in the above condition represents the wavelength of the detection beam in outer air (or vacuum). That is, the difference-h in depth is set within (2m+1)/4 times xc2x10.05xcex the wavelength xcex of the detection light in outer air (or vacuum). When the position detection mark is covered with a thin film, such as a photoresist, having a refractive index n, xcex in the above condition represents the wavelength of the detection light in the thin film. That is, the difference h in depth is set within (2m+1)/4n times xc2x10.05xcex the wavelength xcex of the detection light in the photoresist (in other words, in outer air vacuum). Note that, when the position detection mark is covered with a plurality of thin films, the difference h in depth is set within (2m+1)/4 times xc2x10.05xcex the wavelength xcex of the detection light in a thin film nearest the mark. That is, in the present invention, the wavelength xcex in the above condition of the difference in depth is the wavelength or the center wavelength of the detection light in a medium (thin film, fluid, vacuum, or the like) brought into contact with the position detection mark.
Note that, in the present invention, the condition of the difference in depth is examined in terms of a reduction in detection error caused by the asymmetry of the position detection mark in the direction of height (depth). The amount of a diffracted beam is maximized in the phase gratings having difference in depth under the above condition of the difference in depth to greatly reduce the influence of the asymmetry of the position detection mark in the direction of height (depth) on the position detection error.